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A tsunami is a very long-wavelength wave of water that is generated by sudden displacement of the seafloor or disruption of any body of standing water. Tsunami are sometimes called seismic sea waves, although, as we will see, they can be generated by mechanisms other than earthquakes. Tsunami have also been called tidal waves, but this term should not be used because they are not in any way related to the tides of the Earth. Because tsunami occur suddenly, often without warning, they are extremely dangerous to coastal communities.

Physical Characteristics Of Tsunami

All types of waves, including tsunami, have a wavelength, a wave height, an amplitude, a frequency or period, and a velocity.

Wavelength is defined as the distance between two identical points on a wave (i.e. between wave crests or wave troughs). Normal ocean waves have wavelengths of about 100 meters. Tsunami have much longer wavelengths, usually measured in kilometers and up to 500 kilometers.

  • Wave height refers to the distance between the trough of the wave and the crest or peak of the wave,

  • Wave amplitude refers to the height of the wave above the still water line, usually this is equal to 112 the wave height. Tsunami can have variable wave height and amplitude that depends on water depth as well as moment. Wave frequency 0 period Is the amount of time it takes for one full wavelength to pass a stationary point.

  • Wave velocity is the speed of the wave. Velocities of normal ocean waves are about 90 km/hr while tsunami have velocities up to 950 km/hr (about as fast as jet air planes), and thus move much more rapidly across ocean basins.

Tsunami are characterized as shallow water waves. These are different from the waves most of us have observed on the beach, which are caused by the wind blowing across the oceans surface. Wind-generated waves usually have period (time between two successive waves) of five to twenty seconds and a wavelength of 100 to 200 meters. A tsunami can have a period in the range often minutes .to two hours and wavelengths greater than 500 km. A wave is characterized as a shallow water wave when the ratio of the water depth and wavelength is very small. The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wavelength, it will lose little energy as it propagates. Thus, in very deep water, a tsunami will travel at high speeds with little loss or energy. For example, when the ocean is 6100 m deep, a tsunami will travel about 890 km/hr, and thus can travel across the Pacific Ocean in less than one day. As a tsunami leaves the deep water of the open sea and arrives at the shallow waters near the coast, it undergoes a transformation. Since the velocity of the tsunami is also related to the water depth, as the depth of the water decreases, the velocity of the tsunami decreases.

The change of total energy of the tsunami, however, remains constant. Furthermore, the period of the wave remains the same, and thus more water is forced between the wave crests causing the height of the wave to increase. Because of this shoaling effect, a tsunami that was imperceptible in deep water may grow to have wave heights of several meters or more. If the trough of the tsunami wave reaches the coast first, this causes a phenomenon called drawdown, where it appears that sea level has dropped considerably. Drawdown is followed immediately by the crest of the wave which can catch people observing the drawdown off guard. When the crest of the wave hits, sea level rises is called run-off. Run-up is usually expressed in meters above normal high tide. Run-ups from the same tsunami can be variable because of the influence of the shapes of coastlines. One coastal area may see no damaging wave activity while in another area destructive waves can be large and violent. The flooding of an area can extend inland by 300 m or more, covering large areas of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat. Tsunami may reach a maximum vertical height on shore above sea level, called a run-up height, of 30 meters. A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 60 meter high wave.